The pollution question was added to the 1971 survey after Lake Erie was declared a “dead lake” due to industrial pollution in 1970 and the Cuyahoga River was so polluted it set on fire in 1969. This created a national outcry and resulted in the Great Lakes Water Quality Act and Clean Water Act. In turn, responses to the pollution question began with a very high percentage: 44.4% in 1971 and 46.3% in 1972. After this peak in attention, there was a 12 point drop in 1973 and the downward trend continued.

During the Gen X time period, there was another boost in the results. In 1990, 34.4% of respondents rated the pollution question highly, a 5.6 point boost from the year before. The reason: the Exxon/Valdez oil spill in 1989. Again, this resulted in a boost in environment concern for a couple years that then dropped off over time.

Since Twenge’s analysis ends in 2009, the Millennial generation’s event that increased attention of pollution, the BP Deepwater Horizon spill, is not included.

Twenge using the mean of each generation’s responses results in the perception that the Boomers and Gen X had much higher sustained concern about pollution, when in fact they both benefited from outlier years that raised the mean.

As you mentioned in the post, the updated question in 2011 resulted in 40.8% of Millennials rating it as a top concern, close to the post-Erie Boomer number.